Multistage submersible pumps often are used in water wells to supply water for residential, commercial and agricultural uses. The wells encountered may be quite deep rendering it necessary to employ a pump capable of developing great pressure in order to force liquid to the surface. Among the factors which control the amount of liquid output is the relatively small diameter of the well casing through which must pass the entire pump including external housing, motor, and the many stages which contact and move the liquid. To compensate for the limited diameter, numerous stages are stacked within the housing, resulting in a pump of considerable length, to achieve a lift of several hundred feet.
A typical prior art, multistage pump is disclosed in U.S. Pat. No. 3,779,668 entitled "Stage For A Centrifugal Pump". Such prior art pumps employ a continuous exterior pump housing, or casing, into which are stacked the various internal components which constitute the pump stages. The casing is closed at its lower and upper ends by a suction bowl and discharge bowl, respectively. A central pump shaft, coupled at one end to a motor, extends axially through the pump casing and operatively engages each impeller.
As can be appreciated, several inherent drawbacks exist in these prior art pumps. Specifically, the pump casing proves to be a costly feature of the pump both in manufacturing--it must be manufactured to exacting tolerances of concentricity throughout its length--and in operational efficiency--its diametrical size further restricts the size of the impeller and the area available to pump the fluid. Additionally, because the internal components merely are stacked within the pump casing, interstage leaking commonly occurs, that is, fluid from one stage leaks back to a preceding stage reducing the overall efficiency of the pump.
It also should be appreciated that a particular casing is suitable only for a specific number of stages. Whenever a pump of more or less stages is desired, it is necessary to manufacture a specific casing to house the particular number of stages. Furthermore, as the number of stages increase, the cost of manufacturing a casing to accommodate the stages increases disproportionately.